Cellular resistance to DNA damage and replication errors is critical to survival of cells, tissues, and organisms. Radiation induces DNA strand breaks. Failure to repair even one DNA strand break can be lethal in yeasts. Cellular resistance to DNA damage consists of separate processes for recognition of damage and repair Control mechanisms exist for arresting the cell division cycle (cdc) until DNA repair is completed. Delay can occur in different phases of the cell cycle depending on the type of DNA damage and the stage in the cell cycle at which the damage occurs. In particular, damage resulting from DNA strand breaks caused by ionizing radiation or topoisomerase inhibitors causes delay of the cell cycle in the G2 phase before entry into mitosis. The delay may be observed as a decline in the mitotic index of human or yeast cells approximately one hour post irradiation.
Several classes of mutations in yeasts have been defined that result in deregulation of the cell cycle. Temperature-sensitive (ts) mutations in yeast cdc genes can result in death at defined points in the cell cycle when strains are shifted to the non-permissive temperature, and lethality may increase in a temperature-sensitive manner (1). More than thirty-two different cdc genes have been identified in S. cerevisiae (2). One such mutant, cdc9-8.sup.ts, is a DNA ligase mutant in which the temperature-dependent increase in lethality presumably occurs because of a general failure in ligating chromosomal DNA Okazaki fragments following chromosomal DNA replication. The molecular activities of most cdc genes is largely unknown.
Recently a new class of cell cycle regulatory mutations has been identified and labeled checkpoint mutations (3). Checkpoints exist to ensure that DNA synthesis is completed before mitosis begins; that anaphase is delayed until all the chromosomes arrive on the metaphase plate; that centrosome duplication does not occur until DNA has been synthesized; and, that initiation of DNA synthesis is coordinated between different regions in a chromosome. In yeast, RAD9 is one such checkpoint gene of S. cerevisiae that mediates G2 delay after DNA damage. rad9 mutants have greatly increased radiation sensitivity (less than 0.1% survival at 8000 rads for rad9 yeasts vs. 30% for RAD+ yeasts) (4). Direct visualization of budding yeasts after irradiation shows that rad9 cells continue into mitosis despite potentially lethal DNA damage and die in subsequent generations. RAD9 protein is not required for DNA repair, and RAD9 is not an essential gene in the cell cycle. In the absence of DNA damage, rad9 cells display normal cell cycle kinetics but accumulate spontaneous chromosome loss at a higher rate than wild-type strains. Northern blot analyses of RNA from yeast in different parts of the cell cycle and from pre- and post-irradiated cells show a constant level of RAD9 MRNA. The yeast RAD9 gene has been cloned, and the translated open reading frame encodes 1309 amino acids that exhibit no significant homology to any other known proteins in the database (4). No human genes have been identified that mediate the G2 delay induced by DNA damage.
The simultaneous presence of both a rad9 checkpoint mutation and a cdc9-8 mutation (i.e., in a double mutant strain) substantially increases the rate of cell death when cells are shifted to the nonpermissive temperature (4). This increase in lethality is presumably due to DNA strand breaks resulting from incomplete DNA synthesis (cdc9-8) and failure to properly delay the cycle to repair the damage (rad9).
CDC34 (not to be confused with p34.sup.cdc2) is an essential gene in yeast required for the transition from late G1 to the initiation of DNA synthesis (5). Sequence analysis and enzymatic assays support the notion that CDC34 is an E2 ubiquitin ligase. The target protein ubiquitinated by CDC34 is unknown.
While it has been possible to study checkpoint genes in yeast, few of their human counterparts have been identified and it is not presently known whether events observed in yeast will be generally applicable to cell cycles of higher eukaryotes.